The present invention relates to an ink jet recording method and apparatus using an ink jet recording head having a plurality of ejection outlets (nozzles), capable of tone recording.
In an ink jet recording system using the recording head, the ink is ejected to a recording material in accordance with recording signals. The system is widely used because of the low running cost and the quietness. A great number of nozzles are arranged in a line extending perpendicular to the relative movement direction between the recording material and the recording head, and therefore, one scan of the recording head over the recording material can cover a recording width corresponding to the number of nozzles, so that the high speed recording is accomplished relatively easily.
When a tone gradation is to be provided in the ink jet recording system, it will be considered to change the size of liquid droplet ejected. However, there is no practical method for accomplishing this. Usually, therefore, the number of ink droplets per unit area is controlled on the basis of pseudo-half-tone image processing. In another method called xe2x80x9cmulti-droplet systemxe2x80x9d, a smaller size ink droplet is used, and a plurality of such ink droplets are deposited substantially at the same point on the recording material to provide one recorded dot, in which the number of ink droplets is changed to reproduce the tone. This system permits the tone recording without reduction of the image resolution, and is particularly effective for the ink jet recording system in which difficulties arise in significantly changing the size of one liquid droplet.
However, in a conventional multi-droplet system, one picture element (pixel) is recorded by a plurality of ink droplets ejected from one and the same nozzle, and therefore, if there is a variation in the sizes of the ink droplets of the individual nozzles, i.e., each individual nozzle ejecting an ink droplet having the same nominal volume/size but different actual volume/size than an ink droplet ejected from other nozzles, a non-uniform image results which includes stripes and/or image density unevenness.
This problem becomes more significant where the number of nozzles of a recording head is increased to expand the recording width covered by one scan in an attempt to accomplish the high speed recording. The increase of the nozzle number and therefore the recording width results in a greater component of the spatial frequency of the unevenness, and therefore, in more conspicuous unevenness. Thus, the image quality is degraded. In the case of the tone recording, the unevenness is so conspicuous that only a several % variation among the ejection quantities of the nozzles is enough for one to recognize stripes caused by the density unevenness.
In order to avoid the problem, the conventional multi-droplet system requires very accurate head manufacturing in order to reduce the variation in the ejection quantities through the individual nozzles. This brings about high cost and low yield. As a method for removing the density unevenness through software, it is effective to change the number of ink ejections to compensate for the variation of volume/size of ejected ink droplets among the nozzles with the aid of image processing of error diffusing method or the like. However, such an image processing system results in increase of the system cost. In addition, even if such a processing is used, the number of ink droplets has to be readjusted if the variation among nozzles in the ink volumes changes with time. This makes the maintenance operation difficult. Furthermore, this method does not work where there is a non-ejection nozzle.
This system also involves the problem that the density unevenness is not sufficiently suppressed when the variation in the ink droplet volume is larger.
In order to accomplish a high quality tone recording of not less than 16 tone gradations in the above-described system, stabilized ink ejections with very small droplets are required. Therefore, the manufacturing accuracy of the recording head has to be very high, so that the manufacturing method is totally different from that for the bi-level recording heads. This results in high cost and low yield.
In the case of multi-droplet system of 3-5 tone gradations, the droplet size, volume or quantity is permitted to be relatively large as compared with that in the case of the 16 or more tone gradations. Therefore, the manufacturing tolerance in the recording head is so large that the same manufacturing method as in the bi-level recording head can be used. The cost can be reduced.
The image provided by the recording head having such a large number of tone gradations is better in the image quality than the image recorded by the bi-level recording head because the grains are not conspicuous. However, as compared with the image provided by the recording head having 16 or the like tone levels, particularly in the grains in the high light portions.
U.S. Pat. No. 4,746,935 proposes multi-tone ink jet printer capable of accomplishing the tone recording on the basis of combinations of 1 pl, 2 pl and 4 pl, for one pixel. According to this proposal, 8 kinds of ink droplet volumes, i.e., 0, 1, 2, 1+2(=3), 4, 1+4(=5), 2+4(=6), 1+2+4(=7), can be provided by three kinds of ink droplets (volume ratio). Therefore, the printing speed is increased as compared with the case where one ink droplet is overlaid 7 times. However, as shown in FIG. 2 of the U.S. ""935 patent the curve representing the relationship between the reflection density and the total volume of the ink droplets for one pixel is steep and convex-up. For this reason, even if the differences between adjacent total volumes of the ink for one picture element are the same, the differences, in the reflection density, corresponding thereto, are not the same. Therefore, in the zone where the volume of the ink droplet for one pixel is small, the differences of the adjacent possible reflection densities is large. On the contrary, in the zone where the volume of the ink droplet for one pixel is large, the difference between the possible reflection densities is small. In other words, the volume of the ink droplet does not significantly influence the tone gradation in the zone where the volume of the ink droplet for one pixel is large. In addition, since the number of combinations of different ink droplets for one picture element is large (8 combinations in the case of 1 pl, 2 pl and 4 pl), the image processing circuit becomes complicated with the result of high design and manufacturing cost.
Another problem of the ink jet printer of the U.S. patent is that one recording head has to be provided with the nozzles having different ejection volumes, the difference being as large as 4 times (4 pl/l pl), or 8 times in the case of 1 pl to 8 pl. In this case, the difficulties arise in the manufacturing of the recording head. Generally, the recording head parameters influential to the volume of the ink droplet ejected, a distance between the heater and the ejection outlet, a size of the heater, a configuration of the ink outlet or barrier and an ejection outlet area. In order to change the volume of the ink droplet from 1 to 4, the changes of the heater-outlet distance, the heater area and the ejection outlet area has to be changed. The manufacturing will be difficult only using the conventional practical method. Therefore, in order to accomplish such a recording head, a new process has to be added, with the result of increase of the manufacturing cost.
In the ink jet printer disclosed in the U.S. ""935 patent, the ink ejection outlets providing the different ejection volumes (1 pl, 2 pl, and 4 pl) are arranged along a scanning direction of the recording head and closely with each other at the front side of the recording head, so that the plural ink droplet ejections for a given one pixel can be effected through one scan. Therefore, the ink droplets are sequentially overlaid before the previous ink droplet has not yet seeped into or fixed on the recording material. In the image region in which the number of overlying droplets is large, the adjacent pixels are in contact with the result of feathering. If this occurs, the characters or the like become less clear. In the case of color image, the edges of the image becomes blurred by the feathering and ink mixture adjacent the edge of the monochromatic region, with the result of significant problem of the unacceptable degradation of the record quality.
In the case of color image in the ink jet recording head, there is a problem that the edges of the image is blurred due to the smear resulting from ink mixture before the fixing at the edge of the monochromatic region, particularly. In order to avoid this problem, in the pixel area modulating method such as dithering method, there are known methods in which special recording material having a coated layer of high ink-absorbing nature to prevent the color mixture for individual dots, or in which different color dots are arranged in a staggered fashion as a preventing method for individual picture elements. However, if such method as used as they are, the running cost for the image output is increased, or the image resolution is decreased due to the staggered arrangement.
The feathering or expansion of the ink in the recording material can be reduced by providing a certain length of fixing period. As a method using this, Japanese Laid-Open Patent Application No. 4523/1990 proposes that the recording material is scanned on the same line plural times, while the recording material is at rest, the number of scans being larger than the number of required colors. However, with this method, when black, yellow, magenta and cyan ink materials are used, the required time is 4 times with the result of significant reduction of the output speed.
On the other hand, in order to provide a wide tone gradation range with the multi-droplet system, the adjacent liquid droplets are not in contact with the result of less expansion of the liquid when the number of liquid droplets overlaid on the recording material is small, although a sufficiently small liquid droplet is required as compared with the bi-level recording. However, where the number of liquid droplets overlaid is large, the adjacent liquid dots are in contact with the result of the large expansion or feathering.
Accordingly, it is a principal object of the present invention to provide an ink jet recording method and apparatus in which the tone recording is improved.
It is another object of the present invention to provide an ink jet recording method and apparatus in which the tone recording is improved even if the ink droplet volumes are varied among the nozzles.
It is a further object of the present invention to provide an ink jet recording method and apparatus in which the tone recording is improved even if one or some nozzles failed.
It is a yet further object of the present invention to provide an ink jet recording method and apparatus in which uniform tone gradation can be provided despite property change of the recording head with time.
It is a yet further object of the present invention to provide an ink jet recording method and apparatus wherein the variations in the volumes of the ink droplets ejected through individual nozzles is reduced for any tone gradation to suppress the unevenness of the image.
It is a yet further object of the present invention to provide an ink jet recording method and apparatus in which the high quality tone recording without conspicuous grains is possible without extremely reducing the ink droplet volume.
It is a further object of the present invention to provide an ink jet recording method and apparatus in which a large number of tone gradation levels can be provided with a small number of droplets.
It is a yet further object of the present invention to provide an ink jet recording method and apparatus in which the expansion or feathering of the image dot is suppressed to provide desired colors of the image.
According to an aspect of the present invention, there is provided a liquid jet recording method of recording on a recording material with liquid droplets discharged through plural scanning nozzles, the improvements residing in:
n/(s/p)xe2x89xa72 
n/(s/p)xc3x97k=gxe2x88x921 
are satisfied, where n (nxe2x89xa72) is a number of the nozzles arranged at pitch P (xcexcm); s (xcexcm) is a distance of relative movement between the nozzles and the recording material between adjacent scans; k (kxe2x89xa71) is a maximum number of ink droplets per pixel and per scan; g (gxe2x89xa73) is a number of tone levels.
Then, one pixel is recorded by m nozzles through m main scans, where m=n/(s/nozzle pitch). As a result, when the ink droplet volume variation among the nozzles is in the form of the normal distribution with a standard deviation a, for example, the variation of the ink quantity per pixel is reduced to "sgr"/{square root over (m)}, since one pixel is recorded by different m nozzles.
According to another aspect of the present invention, there is provided a liquid jet recording method of recording on a recording material with liquid droplets discharged through plural scanning nozzles, the improvements residing in:
n/(s/p)xe2x89xa72 
n/(s/p)xc3x97k greater than gxe2x88x921 
are satisfied, where n (nxe2x89xa72) is a number of the nozzles arranged at pitch P (xcexcm); s (xcexcm) is a distance of relative movement between the nozzles and the recording material between adjacent scans; k (kxe2x89xa71) is a maximum number of ink droplets per pixel and per scan; g (gxe2x89xa73) is a number of tone levels.
Then, s/(nozzle pitch)=t represents the relative movement distance between the recording head and the recording material by the sub-direction scan (sheet feed amount in the case of a serial printer) on the basis of a distance between adjacent nozzles. The number of scans for one picture element is m or m+1, where n/t=m (the decimal fraction is neglected, and in the case of no decimal fraction, the number of scans is m for one picture element).
The maximum number of ink droplets for one pixel is (m+1)xc3x97k, or mxc3x97k (where n/t includes no decimal fraction), but the number of droplets per one pixel is 0xe2x88x92gxe2x88x921. Therefore, according to this aspect of this invention, the capacity of the number of ink droplets is larger than the number of ink droplets to be supplied to one pixel, and therefore, even if one or some of the ejection nozzles failed, they can be compensated for by other nozzles, so that the image is maintained uniform.
According to a further aspect of the present invention, there is provided a liquid jet recording method in which plural liquid droplets are deposited at substantially the same portion to record a tone image, the improvement residing in that a plurality of nozzles for discharging the droplets are prepared, and the nozzles are operated to discharge the droplets in accordance with a scheme determined to operate the nozzles at substantially even frequencies.
Then, the actuation frequencies of the individual nozzles are more uniform, and therefore, the limited number of nozzles actuation does not occur.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.